1. Field of the Invention
This invention relates to synthetic hectorite coated films. More particularly, this invention relates to a coating composition, especially a two-component coating composition, for coating plastic film, wherein the coating comprises synthetic hectorite of a particular type and an air curable hydrophilic epoxy ester binder so as to obtain a coating which can be applied to a plastic substrate, thereby enhancing one or more properties especially the antistatic properties.
2. Description of Related Art
Synthetic hectorite, commercially available as Laponite from Southern Clay Products, Inc., a subsidiary of Laporte Industries Ltd., has been widely used for a wide variety of purposes including its use for imparting antistatic properties to various materials.
One field where antistatic properties are desireable is in the use of packaging films, particularly packaging films formed from plastic materials. Films of this type are extremely well known in the art and include a wide variety of polyolefin films, in particular polypropylene and polyethylene films, as well as other plastic films such as polystyrene films, polyester films, polycarbonate films, polyamide films, and fluoropolymer films.
There have been two general procedures which have been employed in the art in order to impart antistatic properties to various plastics, the first being the inclusion of various inorganic minerals, including hectorite, into the plastic prior to forming it into a film. The second approach has been to coat plastic films with a coating composition containing various inorganic materials, including synthetic hectorite.
There have been many problems associated with coating films in order to provide a protective layer which has desirable properties and certain of these disadvantages are set forth in U.S. Pat. No. 4,786,588. As is disclosed in this patent, protective layers containing inorganic substances can have insufficient covering properties and the surface of the layer can become rough and/or the coating can flake off during use. One approach towards solving these problems is disclosed in U.S. Pat. No. 4,786,588 in that the inorganic material which is used is modified by treating it with various ions, etc.
A second approach taken in the prior art is disclosed in Japanese publication H3-169540 wherein the plastic films are coated with Laponite and various non-epoxy water-soluble binders and adhesion and other desirable properties are alleged to be improved by the inclusion of kaolin. The Japanese publication teaches that without the addition of kaolin, adequate materials are not prepared.
A third approach is disclosed in U.S. Pat. No. 4,868,048 wherein certain fractions, i.e., neighborite is removed from synthetic hectorite before use thereof as a coating with a non-epoxy binder.
The use of synthetic hectorite as a coating for paper is disclosed in U.S. Pat. No. 4,173,480. According to this patent, photographic sheet material is prepared by sizing paper with gelatin, starch or carboxymethylcellulose, a synthetic hectorite clay, specifically Laponite S, and a polymeric film is overlaid onto the paper.
There is a continuing need to provide an improved coating for plastic films exhibiting improved properties, particularly antistatic properties without impairing other important properties such as blocking and transparency, and wherein the coating firmly adheres to the film and does not separate or flake off during or after processing.